Intimate link between ammonium loss of phengite and the deep Earth's water cycle

Abstract

Phengite, a high-pressure white mica, is stable down to ∼300 km depth in subduction environment and represents an important carrier of nitrogen and hydrogen to the deep Earth. Although the determination of nitrogen and hydrogen recycling efficiency to subduction zones is crucial for building global geodynamic models of volatile elements, no experimental data have so far evaluated the rate of ammonium release from phengite and its potential effect on phengite dehydration in a descending slab. Here, we investigated the kinetics of ammonium loss and dehydration of ammonium-bearing and ammonium-free phengite samples using FTIR spectroscopy. Single phengite grains were analyzed after annealing at high temperatures from 720 to 850°C for different periods of time. The results show that ammonium loss is one order of magnitude faster than dehydration at the same temperature (for instance, with diffusion coefficients of 1.5×10−15 m2/s and 1.0×10−16 m2/s at 800°C for ammonium loss and dehydration, respectively). Moreover, dehydration of ammonium-bearing phengite is one order of magnitude faster than that of ammonium-free phengite (with diffusion coefficients of 1.0×10−16 m2/s and 2.0×10−17 m2/s at 800°C for dehydration in ammonium-bearing and ammonium-free phengite, respectively). The activation energies of ammonium loss, dehydration in ammonium-bearing phengite and dehydration in ammonium-free phengite are around 150, 535, 548 kJ/mol, respectively. These results imply that ammonium release from phengite is not only easier than dehydration but also can promote dehydration. The geochemical behavior of ammonium in phengite is consistent with previous observations on natural samples from subduction zones of different geothermal gradients. Furthermore, since ammonium release can trigger dehydration, water may be released at depth locus shallower than theoretical phengite breakdown (i.e. <300 km). Thus, ammonium present in hydrous minerals may exert a strong control on the water flux released in subduction zones.